An LCD has the advantages of portability, low power consumption, and low radiation. LCDs have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
A typical LCD includes an LCD panel, a backlight for illuminating the LCD panel, and a backlight control circuit for controlling the backlight. The backlight control circuit includes a pulse generator configured for generating a square pulse, a backlight modulation circuit configured for generating a backlight adjusting signal according to the square pulse, and an inverter circuit configured for transforming a low direct current (DC) voltage to a high alternating current (AC) voltage. The high AC voltage drives the backlight according to relative duty ratios of the backlight adjusting signal. The backlight can include one or more lamps, such as cold cathode fluorescent lamps.
FIG. 5 is a diagram of a typical backlight modulation circuit used in a backlight control circuit of an LCD. The backlight modulation circuit 100 includes a pulse generator 110, an integrating circuit 120, a voltage division circuit 130, an oscillator circuit 140, an amplifier 150, and a regulation circuit 160.
The amplifier 150 includes a negative input, a positive input, and an output.
The oscillator circuit 140 includes a low frequency oscillator 143 and a capacitor 141. The low frequency oscillator 143 is connected to ground via the capacitor 141. An electrical connecting node between the low frequency oscillator 143 and the capacitor 141 is connected to the positive input of the amplifier 150. A capacitance of the capacitor 141 is approximately 4.7 nF (nanofarads).
The pulse generator 110 includes a scaler 111, an NMOSFET (n-channel metal-oxide-semiconductor field-effect transistor) 112, a bias resistor 113, and a 5V (volts) DC power supply 114. The NMOSFET 112 includes a source electrode “S” connected to ground, a drain electrode “D” connected to the power supply 114 via the bias resistor 113, and a gate electrode “G” connected to an output of the scaler 111 for receiving a pulse signal therefrom.
The integrating circuit 120 includes an integrating resistor 121 and an integrating capacitor 122. The drain electrode “D” of the NMOSFET 112 is connected to ground via the integrating resistor 121 and the integrating capacitor 122 in series. A resistance of the integrating resistor 121 is approximately 47Ω (ohms). A capacitance of the integrating capacitor 122 is approximately 0.1 μF (microfarads).
The voltage division circuit 130 includes two voltage division resistors 131, 132. An electrical connecting node between the integrating resistor 121 and the integrating capacitor 122 is connected to ground via the voltage division resistor 131 and the voltage division resistor 132 in series. An electrical connecting node between the two voltage division resistors 131, 132 is connected to the negative input of the amplifier 150. A resistance of the voltage division resistor 131 is approximately 100 KΩ (kiloohms). A resistance of the voltage division resistor 132 is approximately 47 KΩ.
The regulation circuit 160 includes a current limiting resistor 161, a filter capacitor 162, and a 5V DC reference power supply 163. The reference power supply 163 is connected to ground via the current limiting resistor 161 and the filter capacitor 162 in series. An electrical connecting node between the current limiting resistor 161 and the filter capacitor 162 is connected to the negative input of the amplifier 150.
The pulse generator 110 outputs a square pulse at the drain electrode “D” of the NMOSFET 112. This square pulse is shown in FIG. 6. An amplitude of the square pulse is approximately 5V. Then the integrating circuit 120, the voltage division circuit 130, and the regulation circuit 160 transform the square pulse signal to a 1.5V DC voltage. This 1.5V DC voltage is shown in FIG. 7. Then the regulation circuit 160 provides the 1.5V DC voltage to the negative input of the amplifier 150. The oscillator circuit 140 is configured to generate a triangular pulse (as shown in FIG. 8), and provide the triangular pulse to the positive input of the amplifier 150. An amplitude of the triangular pulse is approximately 1.5V. The amplifier 150 is configured to output a backlight adjusting signal to an inverter circuit (not shown).
Because the backlight modulation circuit 100 includes the integrating circuit 120, the voltage division circuit 130, and the regulation circuit 160, the backlight modulation circuit 100 is somewhat complicated. Furthermore, the 5V square pulse outputted from the pulse generator circuit 110 is transmitted to the positive input of the amplifier 150 via the integrating circuit 120, the voltage division circuit 130, and the regulation circuit 160 in series. Thus interference may occur when the 5V square pulse is transmitted to the amplifier 150.
It is desired to provide a new backlight modulation circuit which can overcome the above-described deficiencies.